(a) Field of the Invention
The present invention relates to a hydraulic pump for producing working hydraulic pressure for operation of various kinds of hydraulic apparatus, such as an anti-lock brake controller of vehicles.
(b) Description of Prior Art
In general, in vehicles, particularly small-sized vehicles such as motorcycles, it is quite effective in improving the dynamic performance of the vehicle to adopt a so-called unloader system which is adapted to unload the engine by disconnecting the hydraulic pump when a predetermined pump outlet pressure is obtained.
One of the conventional means of utilization of the hydraulic pump as the pressure source of the anti-lock brake control system of the vehicle employs a control hydraulic pressure chamber disposed in the actuator of the wheel brake and adapted to apply a back pressure to the actuator thereby to reduce the braking torque produced by the wheel brake, and solenoid valves disposed in respective hydraulic passages between the control hydraulic pressure chamber and an oil reservoir and between the control hydraulic pressure chamber and an accumulator and adapted to open and close these passages selectively. In this case, the hydraulic pump is disposed between the oil reservoir and the accumulator to charge the latter with pressurized working fluid or oil.
The hydraulic pump can be driven either by an electric motor or by the engine for driving the vehicle. The former method, however, is not suitable for small-sized vehicles such as motorcycles because the pump consumes the electric power which is impractically large as compared with the capacity of batteries, resulting in an unstable operation of the solenoid control valves and other electric equipment. In the latter method, if the hydraulic pump is continuously driven by the engine, the output power of the engine and, accordingly, the dynamic performance of the engine is deteriorated more or less.